Self referencing heterodyne reflectometer and method for implementing

ABSTRACT

A self referencing heterodyne reflectometer is disclosed which rapidly alternates between a heterodyne reflectometry (HR) mode, in which an HR beam comprised of s- and p-polarized beam components at split angular frequencies of omega and omega+Deltaomega is employed, and a self referencing (SR) mode, in which an SR beam comprised of p-polarized beam components at split angular frequencies of omega and omega+Deltaomega is employed. Alternatively, in SR operating mode the SR beam is replaced by a p-polarized amplitude modulated (AM) beam, operating at two modulated amplitudes of alpha and alpha+Deltaalpha at a single frequency, omega&#39;. When the two measurements are made in rapid succession, using an optical chopper switcher, temperature induced noise in the detector is be assumed to be equivalent. Film phase shift information is derived from the measured phase shift deltaRef/film, generated from the HR beam, and the reference phase shift deltaRef/Sub, generated from the SR/AM beam, which are used for calculating film thickness.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 11/237,225 entitled “Self Referencing Heterodyne Reflectometerand Method for Implementing,” filed Sep. 27, 2005 now U.S. Pat. No.7,545,503. The present application is related to co-pending U.S. patentapplication Ser. No. 11/178,856 entitled “Method for Monitoring FilmThickness Using Heterodyne Reflectometry and Grating Interferometry,”filed Jul. 10, 2005, and co-pending U.S. patent application Ser. No.11/066,933 entitled “Heterodyne Reflectometer for Film ThicknessMonitoring and Method for Implementing,” filed Feb. 25, 2005, bothassigned to the assignee of the present application. The aboveidentified applications are incorporated by reference herein in theirentireties.

BACKGROUND OF THE INVENTION

The present invention relates to reflectometry. More particularly, thepresent invention relates to a reflectometer system and method forobtaining thickness information by measuring phase shift in reflectedsplit frequency signals via heterodyne interferometry. Furthermore, thepresent invention relates to a method and system for using theheterodyned signals from a heterodyne reflectometer for measuring thethicknesses of thin and ultra thin films formed over substrates. Stillmore particularly, the present invention relates to a self referencingheterodyne reflectometer for monitoring of film thickness whichcompensates for detector drift. Additionally, the present inventionrelates to a heterodyne reflectometer which compensates for spuriousnoise generated in the optical measurement components. The presentinvention also relates to a heterodyne reflectometer for in situmonitoring of film thickness.

Due to the increasing demand for ultra precise tolerances in chipfabrication, the physical characteristics of the subsequent layers mustbe very carefully controlled during processing to achieve satisfactoryresults for most applications. Broadly defined, interferometry relatesto the measurement of the interaction of waves, such as optical waves.An interferometer works on the principle that two coherent waves thatcoincide with the same phase will enhance each other while two wavesthat have opposite phases will cancel each other out. One prior artmonitoring system utilizes interferometry for measuring variations insurface profiles, from which feature height information can be inferred.Hongzhi Zhao, et al., in “A Practical Heterodyne Surface Interferometerwith Automatic Focusing,” SPIE Proceedings, Vol. 4231, 2000, p. 301,which is incorporated herein by reference in its entirety, discloses aninterferometer for detecting a phase difference between referenceheterodyne signal, and a measurement signal. Height information relatedto the sharp illumination point on the surface can be inferred from themeasurement. Although the reference and measurement signals aregenerated by beams that are propagated over different paths, this is acommon path interferometer. This approach is sometimes referred to asthe common-axis approach or the normal-axis approach because theincident and reflected beams occupy a common path or axis to a targetlocation, which is normal to the surface being examined.

One shortcoming of the common-path heterodyne interferometers known inthe prior art is that the height information is calculated from anaverage height of the large illumination area of the reference signal.Thus, the accuracy of the results is adversely affected by surfaceroughness. Another limitation of the prior art common axis method isthat it does not measure or calculate an actual thickness parameter fora film layer.

Other attempts in monitoring film thicknesses achieve heterodyning byfrequency modulating the light source. U.S. Pat. No. 5,657,124 to Zhang,entitled “Method of Measuring the Thickness of a Transparent Material,”and U.S. Pat. No. 6,215,556 to Zhang, et al., entitled “Process andDevice for Measuring the Thickness of a Transparent Material Using aModulated Frequency Light Source,” disclose such devices, and areincorporated herein by reference in their entireties. With regard tothese devices, a polarized light beam having a modulated frequency isdirected to the target surface and heterodyne interference signals aredetected from two rays, one reflected off the top surface of a targetand a second from a bottom surface of a target. A thickness isdetermined from the number of beats per modulation period by comparingthe heterodyned interference signals with the linearly modulatedintensity of the light source. The principle drawback of these types ofdevices is that since the heterodyning is achieved by frequencymodulating, the source and thinnest film measurable is limited by itsbandwidth.

Other heterodyne interferometers obtained a heterodyned signal from twoseparate beams, a first beam at a first frequency and polarization, anda second beam at a second frequency and polarization. U.S. Pat. No.6,172,752 to Haruna, et al., entitled “Method and Apparatus forSimultaneously Interferometrically Measuring Optical Characteristics ina Noncontact Manner,” and U.S. Pat. No. 6,261,152 to Aiyer, entitled“Heterodyne Thickness Monitoring System,” which are incorporated hereinby reference in their entireties, disclose this type of interferometer.

FIG. 1 is a diagram of a heterodyne thickness monitoring apparatus inwhich a pair of split frequency, orthogonally polarized beams arepropagated in separate optical paths prior to being mixed andheterodyned, as is generally known in the prior art, for use with aChemical Mechanical Polishing (CMP) apparatus. Accordingly, heterodynethickness monitoring system 100 generally comprises a CMP apparatus, awafer 110 and a measurement optical assembly. Wafer 110 includessubstrate 112 and film 114.

The measurement optical assembly generally comprises various componentsfor detecting and measuring a Doppler shift in the optical frequency ofthe reflected beam, including laser source 140, beam splitter (BS) 144,polarization beam splitter (PBS) 146, beam quarter-wave plate 148, beamreflector 152, beam quarter-wave plate 150, mixing polarizer 143,photodetector 147, mixing polarizer 145, photodetector 149, andsignal-processing assembly 154 electrically connected to the outputs ofphotodetectors 147 and 149, which is in turn connected to thicknessprocessor 160.

In operation, laser diode 140 emits a beam having first linear polarizedlight component 102 at a first wavelength and second linear polarizedcomponent 103 at a second wavelength, but orthogonally polarized to thefirst polarization component. The first and second polarizationcomponents 102 and 103 propagate collinearly to BS 144 where a portionof both components are reflected to mixing polarizer 145 as beams 134and 135 and then to detector 149 as beams 116 and 117, where signal I₂is produced.

The transmitted portions of polarization components 102 and 103propagate to PBS 146 as beams 104 and 105. At PBS 146 component 104follows a first transmission path as beam 106 and passes throughreference quarter-wave plate 148 to reflector 152 and is reflected backthrough quarter-wave plate 148 as beam 122 (orthogonally polarized tobeam 106), where it reflects at PBS 146 to mixing polarizer 143 and onto detector 147 as beam 124.

The second polarization component, from component 105, follows aseparate transmission path, from the first path, as beam 107 and isorthogonally oriented to first polarization component 104 and,therefore, reflects off PBS 146, passes through quarter-wave plate 150as beam 109 and propagates to optically transparent rotatable carrier115. Beam 109 experiences partial reflection at the back surface ofrotatable carrier 115, the interface between substrate 112 and the topsurface of film 114, thereby producing partially reflected beams 111S,111T and 111B, respectively. Each of reflected beams 111S, 111T and 111Bpropagate back through quarter-wave plate 150, are transmitted throughPBS 146 as beams 113S, 113T and 113B and propagate collinearly with beam122 to mixing polarizer 145 as beams 124, 135S, 135T and 135B and thendetected at photodetector 147 as signal I₁. Importantly, I₁ is producedfrom both beam 107, which oscillates at one optical frequency andinteracts the film, and beam 122, which oscillates at another opticalfrequency and that propagates in a second optical path that does notinteract with the film. Signals I₁ and I₂ are compared for finding athickness measurement.

When the measurement beam undergoes an optical path length change, thebeat signal will experience corresponding phase shift. The amount ofphase shift can be determined by comparing the phase of the measurementbeam with the phase of the beam without the optical path length change.The phase shift between the beams can be extrapolated to a distance,from which a thickness may be inferred (or change in thickness) for thetarget sample.

As might be apparent, because signal I₁ is detected from two beamshaving different optical paths, only one of which interacts with thesample, any change in the optical path of either beam will be inferredas a change in the distance to the surface of the film. Furthermore,because only the distance to a single point on the surface of the filmis measured; extraneous factors that interfere with that measurement canbe interpreted as a change in thickness, such as wafer tilt. Therefore,this reflectometer is largely relegated to profile measurements.

SUMMARY OF THE INVENTION

The present invention is directed to a self referencing heterodynereflectometer system and method for obtaining highly accurate phaseshift information from heterodyned optical signals, without theavailability of a reference wafer for calibrations. The heterodynereflectometer is generally comprised of an optical light source withsplit optical frequencies, a pair of optical mixers to generate theoptical beat signal, a pair of optical detectors for detecting andconverting the optical beat signal to electrical heterodyne beatsignals, and a phase shift detector for detecting a phase shift betweenthe two electrical signals.

The self referencing heterodyne reflectometer operates in two modes: aheterodyne reflectometry (HR) mode in which an HR beam comprised of s-and p-polarized beam components at split angular frequencies of ω andω+Δω is employed; and a self referencing (SR) mode in which an SR beamcomprised of p-polarized beam components at split angular frequencies ofω and ω+Δω is employed. A measured phase shift δ_(Ref/film) is derivedfrom the I_(ref) and I_(het) signals detected from HR beam and areference phase shift δ_(Ref/Sub) is derived from the I_(ref) andI_(het) signals detected from SR beam. The measured phase shiftδ_(Ref/film) generated from the beat signals of the HR beam is used forfilm thickness measurements. The SR beam is p-polarized and nosignificant reflection will result from a film surface. The reflectionreturning from the film-substrate interface will not carry any phaseinformation pertaining to the film. Therefore, the reference phase shiftδ_(Ref/Sub) generated from the beat signals of the SR beam is equivalentto that obtained using a reference sample.

By alternating between the HR and SR modes in rapid succession,temperature induced noise and phase drift in the detector can be assumedto be the same as for both measurements. A film phase shift Δφ_(film)can then be calculated from the measured phase shift δ_(Ref/film) andthe reference phase shift δ_(Ref/Sub). In so doing, the temperatureinduced detector noise and phase drift on both detectors is effectivelycanceled out, yielding a temperature independent Δφ_(film).

Since the reference phase shift δ_(Ref/Sub) is not affected by changesin the film, and the substrate does not change, any variation betweensuccessive reference phase shift values is attributable to detectornoise or temperature related phase drift. Unacceptable noise levels canbe detected by monitoring sequential reference phase shift values forchange. The magnitude of phase change between the measurements can thenbe compared to a noise threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the present invention areset forth in the appended claims. The invention itself, however, as wellas a preferred mode of use, further objectives and advantages thereof,will be best understood by reference to the following detaileddescription of an illustrative embodiment when read in conjunction withthe accompanying drawings wherein:

FIG. 1 is a diagram of a heterodyne interferometer as is generally knownin the prior art;

FIG. 2 is a diagram of a heterodyne reflectometer for measuring thinfilm thicknesses;

FIGS. 3A and 3B are diagrams showing the interaction of a linearlypolarized incident beam, comprised of s-polarization component having anoptical angular frequency of ω, and a p-polarization component having asplit optical angular frequency of ω+Δω, with a thin film;

FIGS. 4A-4C are diagrams of operating states of a self referencingheterodyne reflectometer for measuring thin film thicknesses without theavailability of a reference wafer in accordance with an exemplaryembodiment of the present invention;

FIG. 5 is a flowchart depicting the method for finding a film thicknessusing a self-referencing heterodyne reflectometry in accordance with anexemplary embodiment of the present invention;

FIGS. 6A and 6B diagrammatically illustrate the interactions between theHR beam and/or SR beam with the film and substrate;

FIG. 7 is a flowchart depicting the method for identifying detectornoise that may be resistive to the noise canceling in accordance with anexemplary embodiment of the present invention;

FIGS. 8A and 8B are diagrams of a self referencing heterodynereflectometer configured without moving optical components in accordancewith an exemplary embodiment of the present invention;

FIGS. 9A and 9B are diagrams of a self referencing heterodynereflectometer with separate SR beam and HR beam paths in accordance withan exemplary embodiment of the present invention;

FIGS. 10A and 10B are diagrams of a self referencing heterodynereflectometer with counter rotating SR and HR beam paths in accordancewith an exemplary embodiment of the present invention;

FIGS. 11A and 11B are diagrams of a self referencing heterodynereflectometer employing a liquid crystal variable retarder (LCVR) forelectronically switching between HR and SR operating modes in accordancewith an exemplary embodiment of the present invention;

FIGS. 12A and 12B are diagrams of a self referencing heterodynereflectometer in which the SR beam bypasses the sample for addressingthe sole issue of detector phase drift in accordance with an exemplaryembodiment of the present invention;

FIG. 13A and FIG. 13B depicts a self referencing heterodynereflectometer which employ a mechanical or electrical-electromagneticdevice for switching between the HR and SR beams, or vice versa, at afaster rate than the temperature drift in a detector;

FIGS. 14A and 14B are diagrams of a self referencing heterodynereflectometer with counter rotating SR and HR beam paths which utilize ahigh frequency optical switch to minimize error in the phase measurementresulting from changes in the temperature of the detectors in accordancewith an exemplary embodiment of the present invention;

FIGS. 15A and 15B are diagrams of a self referencing heterodynereflectometer, utilizing a chopper, in which the SR beam bypasses thesample for addressing the sole issue of detector phase drift inaccordance with an exemplary embodiment of the present invention; and

FIGS. 16A and 16B are diagrams of a self referencing heterodynereflectometer in which the SR beam path is replaced by an amplitudemodulated (AM) beam in accordance with an exemplary embodiment of thepresent invention.

Other features of the present invention will be apparent from theaccompanying drawings and from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION Element Reference NumberDesignations

100: Heterodyne monitoring system 102: First linear polarized lightcomponent 102: First linear polarized light component 103: Second linearpolarized component 104: First polarization component 105: Secondpolarization component 106: Orthogonally polarized to beam 107: Beam109: Beam 110: Wafer 111s: Reflected beam 111b: Reflected beam 111t:Reflected beam 112: Substrate 113s: Beam 113b: Beam 113t: Beam 114: Film115: Rotatable carrier 116: Beam 117: Beam 122: Beam 124: Beam 134: Beam135: Beam 135S: Beam 135B: Beam 135T: Beam 140: Laser diode 143: Mixingpolarizer 144: Beam splitter 145: Mixing polarizer 146: Polarizationbeam splitter 147: Photo detector 148: Quarter-wave plate 149: Photodetector 150: Quarter-wave plate 152: Beam reflector 154:Signal-processing assembly 160: Data processing system 200: Heterodynereflectometer 202: Split freq. beam with orthogonal, linearly polarizedcomponents 203: Incident beam 204: Split beam 205: Reflected beam 210:Table system 212: Substrate 214: Film 215: Table 220: Light source 222:Optics 224: Beam splitter 232: Incident prism 234: Reflection prism 240:Signal detector 242: Reference signal i_(ref), 250: Signal detector 252:Heterodyne signal i_(ref), 254: Mixing polarizer 255: Mixing polarizer256: Reflected beam 260: Data processing system processor 262: Δφ_(m)measured phase shift detector 264: Memory 266: Δφ corrector 268: D_(f)calculator 269: Film thickness d_(f) 303: Incident s-polarization 303s:S-polarization freq. ω 303p: P-polarization freq. ω and ω + δω 305-1s:Reflected s-ray 305-2s: Refracted s-ray 305-1p: Reflected p-ray 305-2p:Refracted p-ray 400: Light source generates 402: Split freq. beam withorthogonal, linearly polarized components 403: Split freq. beam withorthogonal, linearly polarized components 404: Reflected hr beam 405-1:Reflected beam component 405-2: Reflected beam component 410: Polarizer411: λ/2 plate 412: Beam splitter 413: Aperture 414: Polarizer 416:Reference detector 418: Reflective optical component 420: Reflectiveoptical component 422: Polarizer 424: Focusing optics 426: Optics 426:Measurement detector 433: P-polarized heterodyne beam 434:Self-referencing (SR) beam 435: Reflected SR beam 442: Reference signali_(ref) 452: Heterodyne signal i_(het) 460: Data processing system 461:Slider controller 462: δ_(ref/sub) detector 463: δ_(ref/film) detector465: Threshold noise detector 466: Δφ_(film) calculator 467: Δφ_(film)averager 468: D_(f) calculator 469: D_(f) film thickness 470: Slider800: Light source 801: Beam splitter 802: HR beam 803: HR beam 804:Reference beam 805-1: Ray 805-2: Ray 807: Beam splitter 809: Slidingshutter 810: Stationary polarizer 811: λ/2 plate 812: Beam splitter 815:Polarizer 816: Reference detector 818: Reflective optical component 820:Reflective optical component 822: Polarizer 824: Focusing optics 826:Measurement detector 828: Optical component 829: Optical component 833:Sr beam 834: Beam 835: Ray 900: Light source 901: Beam splitter 902: HRbeam 903: HR beam 904: Reference beam 905-1: Ray 905-2: Ray 909: Slidingshutter 910: Stationary polarizer 911: λ/2 plate 912: Beam splitter 915:Polarizer 916: Reference detector 917: Reflection optics 918: Reflectiveoptical component 920: Reflective optical component 922: Polarizer 924:Focusing optics 926: Measurement detector 928: Optical component 929:Reflective optical component 933: SR beam 934: Beam 935: Ray 1000: Lightsource 1002: HR beam 1003: HR beam 1004: HR beam 1005: Reflected HR beam1010: Stationary polarizer 1011: λ/2 plate 1015: Optics 1016: Detector1018: Beam splitter 1020: Reflective optical component 1021: Reflectiveoptical component 1022: Polarizer 1023: Beam splitter 1024: Focusingoptics 1026: Detector 1033: SR beam 1034: SR beam 1035: Reflected srbeam 1041: Beam splitter 1042 Beam splitter 1050: Corner cube 1051:Shutter 1052: Shutter 1100: Light source 1102: HR beam 1103: HR beam1104: Reference beam 1105: Rays 1107: Beam splitter 1111: Liquid crystalvariable retarder 1112: Beam splitter 1114: Polarizer 1116: Referencedetector 1118: Beam splitter 1119: Polarizing beam splitter 1120:Reflective optical component 1122: Polarizer 1124: Focusing optics 1126:Measurement detector 1128: Optical component 1129: Optical component1133: Beam 1134: Beam 1135: SR ray 1135-1: HR ray 1135-2: HR ray 1200:Light source 1202: HR beam 1203: HR beam 1204: Beam 1205: Reflected HRbeam 1209: Shutter 1212: Optics 1214: Polarizer 1216: Detector 1217:Shutter 1218: Beam splitter 1220: Reflective optical component 1221:Reflective optical component 1222: Polarizer 1226: Detector 1233: Beam1300: Light source 1301: Beam splitter 1302: HR beam 1303: HR beam 1304:Reference beam 1305-1: Ray 1305-2: Ray 1309: Chopper shutter 1310:Stationary polarizer 1311: λ/2 plate 1312: Beam splitter 1315: Polarizer1316: Reference detector 1318: Reflective optical component 1320:Reflective optical component 1322: Polarizer 1324: Focusing optics 1326:Measurement detector 1328: Optical component 1329: Optical component1333: SR beam 1334: Beam 1335: Ray 1361: Chopper controller 1400: Lightsource 1402: HR beam 1403: HR beam 1404: HR beam 1405: Reflected HR beam1410: Stationary polarizer 1411: λ/2 plate 1415: Optics 1416: Detector1418: Beam splitter 1420: Reflective optical component 1421: Reflectiveoptical component 1422: Polarizer 1424: Focusing optics 1426: Detector1433: SR beam 1434: SR beam 1435: Reflected sr beam 1441: Beam splitter1442 Beam splitter 1450: Corner cube 1451: Chopper shutter 1452: Choppershutter 1500: Light source 1502: HR beam 1503: HR beam 1504: Beam 1505:Reflected HR beam 1509: Chopper shutter 1512: Optics 1514: Polarizer1516: Detector 1517: Chopper shutter 1518: Beam splitter 1520:Reflective optical component 1521: Reflective optical component 1522:Polarizer 1526: Detector 1533: Beam 1600: HR light source 1601: AM splitfreq., amplitude mod. beam 1602: HR split freq. beam 1603: HR splitfreq. beam 1604: HR split freq. reference beam 1605-1: AM ray 1605-2: AMray 1609: Chopper shutter 1611: Lght source 1612: Beam splitter 1613:Aplitude modulator 1614: Polarizer 1616: Reference detector 1618:Reflective optical component 1620: Reflective optical component 1622:Polarizer 1624: Focusing optics 1626: Measurement detector 1633: AMamplitude mod. incident beam 1634: AM amplitude mod. reference beam1635: AM amplitude mod. reflected beam D_(f): Film thickness I₁: SignalI₂: Signal I_(het): Heterodyne signal I_(ref): Reference signal

In a Michelson heterodyne interferometer, the interfering reference beamand measurement beam have slight optical frequency difference, typically˜KHz to MHz. The interference between the two is represented by theequation:I=A+B cos(Δωt+φ)  (1)

-   -   A is a direct current component;    -   B is the signal component that represents fringe visibility;    -   φ is the phase difference between reference beam and measurement        beam; and    -   Δω is the angular frequency difference between the two signals.        The interference between the two can be observed as a beat        signal with an angular frequency equal to the difference angular        frequency, Δω.

When the measurement beam undergoes an optical path length change (Δd),the beat signal will experience corresponding phase shift Δφ=(4π×Δd)/λ.

The present inventor has disclosed an uncomplicated heterodynereflectometer approach to thin film measurements in co-pending U.S.patent application Ser. No. 11/178,856 entitled “Method for MonitoringFilm Thickness Using Heterodyne Reflectometry and GratingInterferometry,” filed Jul. 10, 2005, and also in co-pending U.S. patentapplication Ser. No. 11/066,933 entitled “Heterodyne Reflectometer forFilm Thickness Monitoring and Method for Implementing,” filed Feb. 25,2005. In accordance with this approach, the measurement signal isheterodyned from two beam components that each interact with the sample.One of the beam components is almost totally refracted into the film andreflected off the bottom of the film and the other is reflected off thesurface. Thus, the phase of the heterodyned measurement signal is due tothe difference in the optical paths of the two beam components, which,in turn, is related to the thickness of the sample. This concept will beunderstood with the discussion of the heterodyne reflectometer in FIG.2.

FIG. 2 is a diagram of a heterodyne reflectometer for measuring thinfilm thicknesses. Heterodyne reflectometer 200 generally comprisesoptics for directed incident beam 203 incident on film 214 and substrate212 at incidence angle α. Light source 220 generates beam 202 having twolinearly polarized components, operating at split optical frequencies,that are orthogonal with respect to each other for illuminating thetarget. For instance, the beam may have an s-polarized beam component atfrequency ω and a p-polarized beam component at frequency ω+Δω.

Beam 202 is split by beam splitter 224 into beam 204 and beam 203. Beam203 comprises two linearly polarized components that are orthogonal toeach other, with split optical frequencies, i.e., s- and p-polarizedbeam components at split frequencies of ω and ω+Δω, respectively. Asused herein, Δω is approximately 20 MHz, but is merely exemplary andother frequency splits may be used without departing from the scope ofthe present invention. Light source 220 for generating this beam may be,for example, a Zeeman split He—Ne laser. Alternatively, the beam from asingle mode laser source can be split into two separate beams with oneor both of the separate beams being frequency shifted to a predeterminedfrequency using, for example, an acousto-optic modulator. Thesplit-frequency beams can then be recombined prior to incidence withfilm 214. The light beam is directed into the plane of incidence, andtoward film 214, using any suitable optical component for redirectingthe path of the aforementioned light beam. As depicted in the figure, apair of triangular prisms (incident prism 232 and reflection prism 234)direct incident beam 203 to film 214 and receive reflected beam 205 fromfilm 214, but optionally may be any suitable optical component fordirecting the light path while retaining the beam's polarization. Forexample, light source 220 may be directed in the plane of incidence (atincidence angle α from normal), using a mirror or other reflectingoptical component, or, alternatively, coupled into polarizationpreserving fibers which are then positioned to launch the beam at thepredetermined incidence angle.

Notice that the paths of both optical frequencies interact with the filmalong a single path, i.e., the s-polarization component and thep-polarization component of the measurement beam are substantiallycollinear beams and approximately coaxial. Furthermore, the illuminatedareas on film 214 from s-polarization and p-polarization components areapproximately coextensive at the target location.

A primary function of a heterodyne reflectometer of the presentinvention is to determine the actual phase shift, Δφ, from a measuredphase shift, Δφ_(m). Measured phase shift Δφ_(m) is the phase differencebetween the phase of reference signal I_(ref) and the phase ofmeasurement signal I_(het), i.e., the beat of a signal obtained from anon-reflected path (the reference signal) and the beat signal obtainedfrom a reflected path. The true (or actual) phase shift Δφ is necessaryfor determining an error-free and accurate thickness of a film layer,d_(f). Therefore, finding measured phase shift Δφ_(m) necessitatesemploying two signal detectors, one for detecting/generating referencesignal I_(ref) and a second for detecting/generating the measurementsignal I_(het).

Signal detector 240 senses the split beam (reference beam) 204 frommixing polarizer 254, which mixes the s- and p-polarization componentsof beam 204, prior to reflecting off of film 214, and produces referencesignal I_(ref), 242, which is indicative of the phase of beam 204, phaseφ. Detector 240 may be, for example, a PIN (Positive-Intrinsic-Negative)detector, or any photo detector that responds to the beat frequency, andproduces reference signal I_(ref) with a beat frequency of |ω−(ω+Δω)|.Reference signal I_(ref) 242 is transmitted to Δφ_(m) measured phaseshift detector 262, where it is used as the reference phase fordetermining measured phase shift Δφ_(m) induced by film 214.

Signal detector 250, on the other hand, senses reflected beam 256 frommixing polarizer 255, which mixes the s- and p-polarization componentsof beam 205, propagated from prism 234, and after interacting with film214. Signal detector 250 produces measurement signal I_(het), 252, whichis indicative of the phase of beam 256, phase φ+Δφ, and is phase shiftedfrom the phase of reference signal I_(ref) by Δφ. Detector 250 may be,as an example, a PIN detector, which monitors the reflected optical beam256 and produces heterodyne measurement signal I_(het), also with aheterodyne angular frequency of Δω.

Signal 252 is received at Δφ_(m) measured phase shift detector 262,which compares measured heterodyne measurement signal I_(het) 252 withreference signal I_(ref) 242 and determines measured phase shift Δφ_(m).Phase shift Δφ is induced by film 214, and the amount of the phase shiftdepends on several factors, including the thickness of film 214, therefractive index n_(f) for the particular film being monitored, and inhigher phase shifts, a correction factor. The interrelationship betweenthe factors will be discussed in greater specificity further below. Inany case, an accurate film thickness d_(f) 269 can then be determined byprocessor 260 from corrected phase shift Δφ, which is obtained frommeasured phase shift Δφ_(m). However, since measured phase shift Δφ_(m)has an inherent error, at least at higher phase shifts, accuratethickness measurements are possible only after the measured phase shiftis corrected.

Data processed system 260 may take a variety of forms depending on theparticular application. Often data from inline wafer processing isprocessed in real time on a computer or PC that is electrically coupledto reflectometer detectors 240 and 250 or Δφ_(m) measured phase shiftdetector 262. However, the reflectometer systems may be pre-configuredwith internal data processors and/or discrete firmware components forstoring and processing monitored data in real time. Also, the rawmeasured data from the reflectometer may be handled by a data processingsystem resident on the wafer process equipment. In that case, the waferprocessing firmware performs all data processing for the reflectometer,including thickness computations. Accordingly, heterodyne reflectometersystem 200 is depicted with generic data processing system 260, whichmay include discrete firmware and hardware components. These componentsgenerally include measured phase shift corrector 266 and thicknesscalculator 268. Optionally, system 260 may include error correction datamemory 264, the operation of which will be discussed below.

More particularly, Δφ_(m) phase shift detector 262 receives referencesignal I_(ref) 242 and heterodyne measurement signal I_(het) 252 fromthe respective detectors and measures phase shift Δφ_(m) between thetwo. Phase shift detector 262 may use any appropriate mechanism fordetecting corresponding points on reference signal I_(ref) andmeasurement signal I_(het) for phase detection.

Although not depicted in the figure, phase shift detector 262 may alsobe equipped with an I/O interface for entering wavelength and/oroscillator frequency information for facilitating signal detection.

Once measured phase shift Δφ_(m) has been detected, it is passed toΔφ_(m) measured phase shift corrector 266 for error correction. Theerror in measured phase shift Δφ_(m) may be appreciable at higher phaseshifts, but the error can be corrected by applying a polynomial functionto Δφ_(m), with an appropriate set of correction coefficients.Furthermore, Δφ_(m) corrector 266 requires certain parametric data forperforming the error correction computations. These data include thesource wavelength, λ, the top film layer refractive index, n_(f), andthe incidence angle, α. α will be typically set at a default, α=60°,rather than precisely at the Brewster's angle for the source wavelengthand film refractive index n_(f), the reasons for which are discussed inU.S. patent application Ser. No. 11/066,933 “Method for Monitoring FilmThickness Using Heterodyne Reflectometry and Grating Interferometry,”and also in co-pending U.S. patent application Ser. No. 11/066,933entitled “Heterodyne Reflectometer for Film Thickness Monitoring andMethod for Implementing.”

Finally, d_(f) thickness calculator 268 receives the corrected phaseshift, Δφ, from Δφ_(m) corrector 266 and computes a corrected filmthickness d_(f) for the film being examined, i.e., film 214.Alternatively, d_(f) thickness calculator 268 may receive measured phaseshift Δφ_(m) directly from Δφ_(m) phase shift detector 262 and thenalgebraically correct the measured thickness with film thicknesscorrection data it fetches from memory 264. The thickness errorcorrection data, or a look-up table (LUT), are loaded into memory 264beforehand based on the refractive index n_(f) for film 214.

Still another option is to store a table of corrected thickness values,d_(f), in memory 264 which are indexed to discrete measured phase shiftvalues. In that case, on receiving Δφ_(m) from phase shift detector 262,d_(f) thickness calculator 268 retrieves a corrected thickness valuefrom memory 264 and outputs the value.

This method relies on the anisotropic reflection of the radiation fromthe top surface of the film. Therefore, the heterodyne reflectometerset-up is optimally configured with incidence angle α near Brewster'sangle. The maximum sensitivity to phase shift for a film is achieved atthe Brewster's angle for the refractive index of a particular film underexamination. At the Brewster's angle, the amount of reflectedp-polarized light from the top surface of the film is nil or minimal.Thus, signal, I_(het), 252 from detector 250 is rich with film-thicknessinformation.

However, as a practical matter, the optical components in a monitoringsystem may be semi-permanently configured for cooperating with aparticular processing apparatus (e.g., at a preset 60° angle ofincidence, α=60°). In those systems, adjusting the incidence to aprecise angle may be difficult or impossible. Nevertheless, as will beshown in the following discussions, one benefit of the presentlydescribed invention is that the thickness measurements are highlyaccurate over a wide range of angles around the Brewster's angle for aparticular film's refractive index.

Furthermore, in addition to the anisotropic reflection from the filmsurface, reflective anisotropy may also be present in the film itselfand the bottom film surface or the substrate. It has been assumed thatthe film material and the lower interface are isotropic for the s- andp-polarizations. However, this assumption may not always be correct forevery film type, see T. Yasuda, et al., “Optical Anisotropy of Singularand Vicinal Si—SiO₂ Interfaces and H-Terminated Si Surfaces,” J. Vac.Sci. Technol. A 12(4), July/August 1994, p. 1152 and D. E. Aspnes,“Above-Bandgap Optical Anisotropies in Cubic Semiconductors: AVisible-Near Ultraviolet Probe of Surfaces,” J. Vac. Sci. Technol. B3(5), September/October 1985, p. 1498. Accordingly, in those situationswhere the top film and/or the substrate exhibit significant reflectanceanisotropy, the optimized incidence angle can be between normalincidence and Brewster incidence.

The heterodyne reflectometer set-up incidence angle α for configuringsystem 200 is related to, and could change with, the refractive index,n_(f), of the film under inspection and the wavelength, λ, of theillumination source. Since different films have different refractiveindexes, the angle α could be adjusted corresponding to changes in theindex. If this is desired, a means should be provided for adjusting theincident angle of heterodyne reflectometer system 200 based on therefractive index of the various films to be examined. This may beaccomplished by enabling table system 210 and/or prisms 232 and 234 tomove. For example, mirrors 232 and 234 may be configured with twodegrees of movement, one in a rotational direction about an axis that isperpendicular to the plane of incidence formed by beams 203 and 205, andthe normal of film 214, and a translation movement direction that isparallel to the surface normal. Alternatively, mirrors 232 and 234 mayhave one degree of rotational movement about a direction perpendicularto the plane of incidence and table assembly 210 will then have onedegree of translational movement in the normal direction. The latterexemplary embodiment is depicted herein with mirrors 232 and 234 andtable assembly 210 (depicted herein as table 215, film 214 and substrate212) shown with phantom lines indicting movement. The phantom componentsshow incident beam 203 and receiving reflected beam 205 redirected to adifferent incident angle α, in response to a change in the value ofrefractive index n_(f). However, as emphasized above and below, using adefault incidence angle, α=60°, is advantageous over setting theincidence angle precisely at the Brewster's angle for the film and lightsource.

Turning to FIGS. 3A and 3B, the source of phase shift Δφ attributable tofilm 214 is depicted. The s-polarization component of the HR beam isdepicted as being separated (FIG. 3A) from the p-polarization componentof the HR beam (FIG. 3B) for clarity. Turning to the s-polarizationcomponent of the HR beam is depicted in FIG. 3A, incident beam 303 iscomprised of s-polarization component 303 s (having an optical angularfrequency of ω) and p-polarization component 303 p (having an opticalangular frequency of ω+Δω), which are orthogonal to each other. Bothcomponent 303 s and component 303 p are incident to the normal of film214 at angle α. At the surface of film 214, a portion of beam component303 s is reflected as reflected ray 305-1 s, while another portion ofbeam component 303 s refracts into film 314 at a refraction angle, ρ,then reflects off substrate 212 and refracts out of film 214 asrefracted ray 305-2 s. Turning to the p-polarization component of the HRbeam as depicted in FIG. 3B, incident beam component 303 p is split intoa reflected ray 305-1 p and refracted ray 305-2 p.

Basic to calculating accurate film thicknesses is optimizing the lightinteraction with the film to be more sensitive to film thickness, whichin turn enhances the heterodyne phase shift, Δφ_(m). The aim is toincrease the phase shift of the heterodyned signal as much as possiblefrom the reference signal, i.e., increase Δφ_(m). This is done byoptimizing the incidence angle. Since the reflected beam is composed ofs- and p-component rays that are both reflected and refracted, it isadvantageous for one polarization component to have a greater portion ofreflected rays from the film surface than the other. Because s- andp-polarized light with split frequencies is used for the measurement, itis possible to adjust the incident angle, α, to achieve this result. Asis well understood in the art, linear polarized light will exhibit thisresult by setting the incident angle to the Brewster's angle for thesource wavelength. At Brewster's angle, virtually the entirep-polarization component of incident beam 303 p is refracted into thefilm as 305-2 p with very little, if any, reflected as ray 305-1 p.Conversely, operating at Brewster's angle, the s-polarization componentof incident beam 303 s, sees significant reflection as ray 305-1 s withthe rest penetrating the film as refracted ray 305-2 s. Therefore, angleα may be adjusted such that more of one polarized light component is notreflected, but almost totally refracted in the film. Hence, after therays are mixed, the resulting beam is sensitized for phase shift due toa disproportionate contribution of the s-polarization componentreflected from the film's surface. Therefore, it can be appreciated thata phase shift results from the time necessary for refracted componentsto travel over the increased path distance, Δd=2d_(f)√{square root over(n_(f) ²−sin² α)}, where

$\delta_{f} = {\frac{2\pi}{\lambda}\sqrt{n_{f}^{2} - {\sin^{2}\alpha}} \times d_{f}}$

δ is the phase shift attributable to the film thickness;

α is the angle of incidence;

n is the refractive index of the film; and

d is the film thickness.

With the heterodyne reflectometer configured toward being more sensitiveto thickness, a calculation for determining thickness from phase shiftΔφ can be established. In the classical heterodyne interferometer, thephase shift is measured and a corresponding change in the beam pathdifference, Δd, can be calculated using the following expression:Δφ=4π×Δd/λ  (2)

Δφ is the phase shift of the measured signal, I_(het),

Δd is the corresponding beam path difference; and

λ is wavelength of the heterodyne illumination source.

Thus:Δd=Δφλ/4π  (3)

In heterodyne reflectometry, since Δφ=2δ, and

${\delta = {\frac{2\pi}{\lambda}\sqrt{n^{2} - {\sin^{2}\alpha}} \times d}},$the thickness of the film can then be found by the following equation:

$\begin{matrix}{d = \left( \frac{\Delta\;\phi \times \lambda}{4\pi \times \sqrt{n^{2} - {\sin^{2}\alpha}}} \right)} & (4)\end{matrix}$

The proofs of Equations (2)-(4) can be found in U.S. patent applicationSer. Nos. 11/178,856 and 11/066,933 discussed above.

Heterodyne reflectometry by nature is a differential measurementtechnique. In accordance with the prior art, phase shift correspondingto a film is measured with respect to a reference substrate that has afilm of known thickness. Ideally, the operator has access to thereference sample in order to take a reference measurement each timebefore measuring the product/monitor wafer. In the absence of that, onewould require the heterodyne reflectometry sensor to be robust enoughnot to have (systematic) phase drift before the next reference samplemeasurement is made. Highly precise measurements (˜0.001 deg.), areinfluenced by a drift in the heterodyne frequency, phase shift inducedby optical components, presence of surface contaminants, and detectorresponse to temperature change. Some obstacles can be overcome. Becauseof the common mode nature of heterodyne reflectometry, long-termfrequency drift will not influence measurement. Optical componentinduced phase shift can be eliminated by using appropriate coatings andangles of incidence. Taking data in a controlled environment willprevent surface contaminants from influencing measurement. Studies donewith heterodyne reflectometry detectors have shown that phase drift asmuch as 0.01 deg/° C. can occur in a heterodyne reflectometry system ifthe detector temperature is not controlled.

Therefore, in accordance with one aspect of the present invention, aself referencing heterodyne reflectometer and method for implementing isdisclosed. In accordance with another aspect of the present invention, aheterodyne reflectometer and method for implementing is disclosed whichdoes not rely on the availability of reference wafer sample foraccuracy. These aspects of the present invention, as well as otheraspects, will be better understood through a discussion of FIGS. 4A-4Cbelow.

FIGS. 4A-4C are diagrams of a self referencing heterodyne reflectometerfor measuring thin film thicknesses without the availability of areference wafer in accordance with an exemplary embodiment of thepresent invention. FIG. 4A depicts a self referencing heterodynereflectometer showing a composite operational state for obtainingδ_(Ref/film) phase measurements and δ_(Ref/Sub) reference phasemeasurements. FIG. 4B shows the operational state for obtainingδ_(Ref/film) measurements and FIG. 4C shows the operational state forobtaining δ_(Ref/Sub) measurements.

Similar to heterodyne reflectometer 200 discussed in FIG. 2, the selfreferencing heterodyne reflectometer of the present invention generallycomprises optics for directing a beam incident to film 214 and substrate212 at incidence angle α. Light source 400 generates two collinear beams(beam 402) having two linearly polarized components, operating at splitoptical angular frequencies, that are orthogonal with respect to eachother for illuminating the target; an s-polarized beam component atfrequency ω and a p-polarized beam component at frequency ω+Δω. Thisbeam will be referred to hereinafter as a HR (heterodyne reflectometer)beam and will be designated as a soling line in each of FIGS. 4A, 4B,4C, 8A, 8B, 9A, 9B, 10A, 10B, 13A, 13B, 14A and 14B. As used herein, Δωis approximately 20 MHz, but is merely exemplary and other frequencysplits may be used without departing from the scope of the presentinvention. Light source 400 for generating this beam may be, forexample, a Zeeman split He—Ne laser. Alternatively, the beam from asingle mode laser source can be split into two separate beams with oneor both of the separate beams being frequency shifted to a predeterminedfrequency, using for example, an acousto-optic modulator. Thesplit-frequency beams can then be recombined prior to incidence withfilm 214. The light beam is directed into the plane of incidence, andtoward film 214, using any suitable optical component for redirectingthe path of the aforementioned light beam.

HR beam 402 propagates as HR beam 403 and split by BS (beam splitter)412 into reflected HR beam 404, through polarizer 414 (@45°) where areference signal, I_(ref), is detected by detector 416. It should beappreciated that the use of cubes gives rise to certain disadvantages,principally associated with the generation of thermal stress-inducedbirefringence that degrades the polarization performance of thecomponent. Therefore, the beam splitters employed for use with thepresent invention should have low thermal stress-induced birefringence,such as, for example, low linear birefringence SF57 glass. Fused silicacomponents exhibit more thermal birefringence and those with BK7substrate appear to be even less desirable, as their thermalbirefringence properties appear to be on the order of two magnitudesworse than SF57. Thermal stress-induced birefringence should beconsidered in the selection of other optical components, such as mirrorsand the like. Reference signal I_(ref) provides phase information forthe beam before the beam interacts with the sample. The portion of beam403 transmitted through BS 412 propagates off of reflective opticalcomponents 418 and 420 (mirrors or the like) and incident on film 214and substrate 212 (typically a wafer). As discussed above, the angle ofincidence, α, (not shown) is typically set near the Brewster angle forthe source wavelength, λ, of light source 400 and the film's refractiveindex n_(f), the reasons for which are discussed in U.S. patentapplication Ser. Nos. 11/178,856 and 11/066,933 (or at a default, e.g.,α=60°, rather than precisely at the Brewster angle).

HR beam 403 interacts with film 214 and substrate 212 resulting inreflected beam components 405-1 and 405-2, which pass through polarizer422 (@45°) where a heterodyne measurement signal, I_(het), is detectedby detector 426, from focusing optics 424, for the HR beam. As mentionedabove, because this method relies on the anisotropic reflection of theradiation from the top surface of film 214, beam component 405-1 isalmost exclusively s-polarization reflected from the surface of film214, while beam component 405-2 results from interactions below thesurface of film 214. Therefore, beam component 405-2 comprises virtuallythe entire p-polarization component from the incident beam, in additionto some s-polarization component. Film thickness information can beobtained from heterodyne measurement signal I_(het) and reference signalI_(ref) as discussed above with respect to FIG. 2.

When polarizer 410 and λ/2 plate 411 combination (referred tohereinafter as polarizer/λ/2 combination 410/411 or component 410/411)is introduced into the path of beam 402, p-polarized heterodyne beam 433results which is a composite beam made up of both ω and ω+Δωfrequencies. This beam will be referred to hereinafter as a SR(self-referencing) beam.

SR beam 433 is split by BS (beam splitter) 412 into reflected SR beam434, through polarizer 414 (@45°) where reference signal I_(ref) isdetected by detector 416 for the SR beam. The portion of beam 433transmitted through BS 412 propagates off of reflective opticalcomponents 418 and 420 and incident on film 214 and substrate 212following the same path as incident HR beam 403. Incident interacts withfilm 214 and substrate 212 resulting in reflected beam 435, which passthrough polarizer 422 (@45°) where heterodyne measurement signal I_(het)is detected by detector 426 for the SR beam.

When the SR beam is incident on a dielectric film, there is no orinsignificant reflection (˜10⁻³) from the dielectric film surface. Thereflection returning from the film-substrate interface will not carryany phase information pertaining to the film. Therefore, the beatsignals generated by the SR beams can be used to obtain a referencephase value, which is equivalent to that obtained using a referencesample. Thus, because incident SR beam 433 is p-polarized, virtuallynone is reflected from the surface of film 214, but instead interactswith, and is reflected from the interface between film 214 andsubstrates 212. Both the ω and ω+Δω frequency components of thereflected p-polarized SR beam are reflected as an SR beam, beam 435.Consequently, measurement signal I_(het) detected by detector 426 fromSR beam 435 provides a reference phase value that is not affected bychanges in film thickness.

FIG. 4A depicts a composite operational state for generating SR and HRbeams, but as a practical matter the SR and HR beams are propagatedsequentially, with δ_(ref/Sub) and δ_(ref/film) also generatedsequentially. FIG. 4B shows the operational state of the selfreferencing heterodyne reflectometer in the HR beam generation mode fordetecting measurement phase δ_(ref/film). In accordance with thisexemplary embodiment of the present invention, polarizer/λ/2 combination410/411, rather than being stationary, is a sliding optical componentfurther including aperture 413. Sliding polarizer/λ/2/aperturecombination 410/411/413 provides a mechanism for rapidly alternatingbetween an HR beam and an SR beam. In HR beam mode, slidingpolarizer/λ/2/aperture combination 410/411/413 is positioned such thataperture 413 aligns in the path of beam 402, thereby allowing the HRbeam generated by light source 400 to pass. Conversely, in SR beam mode,sliding polarizer/λ/2 aperture combination 410/411/413 is positionedsuch that polarizer/λ/2 combination 410/411 align in the path of beam402, thereby converting HR beam 402 into SR beam 433. The movement forcenecessary is provided by slider actuator 470, which is controlled byslider controller 461.

Continuing, in the HR beam generation mode, slider controller 461instructs slider actuator 470 to move in the HR beam position withaperture 413 aligned directly in the path of beam 402. Incident HR beam403 propagates to detectors 416 and 426 as described above resulting inreference signal I_(ref) and measurement heterodyne signal I_(het).Signals I_(ref) and I_(het) are routed to slider controller 461 which,in turn, switches the path of the signals to δ_(Ref/Sub) detector 462 orδ_(Ref/film) detector 463 depending on the propagation mode; in HR modethe signals I_(ref) and I_(het) are routed to detector 463 and in SRmode the signals I_(ref) and I_(het) are routed to δ_(Ref/Sub) detector462. δ_(Ref/film) is the phase difference between the signals I_(ref)and I_(het) operating in HR mode. Using Equation (5) below, δ_(Ref/film)detector 463 detects δ_(Ref/film) from signals I_(ref) and I_(het).δ_(Ref/film)=(φ_(Ref)+φ_(noise1))−(φ_(het)+φ_(Sub)+φ_(film)+φ_(noise2))  (5)

-   -   Where, δ_(Ref/film) is the phase shift due to the film,    -   φ_(Ref) is the phase shift associated with the reference        detector from BS 412,    -   φ_(noise1) is the phase shift associated with the detector        noise,    -   φ_(het) is the phase shift associated with the heterodyne        measurement detector from BS 412,    -   φ_(noise2) is the phase shift associated with the detector        noise,    -   φ_(Sub) is the phase shift associated with the substrate, and    -   φ_(film) is the phase shift associated with the film.

FIG. 4C shows the operational state for the self referencing heterodynereflectometer in the SR beam generation mode. Here, slider controller461 instructs slider actuator 470 to move in the SR beam position withpolarizer/λ/2 combination 410/411 directly in the path of beam 402,thereby converting HR beam 402 into a p-polarized SR beam with splitoptical frequencies of ω and ω+Δω (SR beam 433). Incident SR beam 433propagates to detectors 416 and 426 as described above resulting inreference signal I_(ref) and measurement heterodyne signal I_(het).Signals I_(ref) and I_(het) are routed to slider controller 461 whichnow switches the path of the signals to δ_(Ref/Sub) detector 462.Reference phase δ_(Ref/Sub) is the phase difference between the signalsI_(ref) and I_(het) operating in SR mode. Using Equation (6) below,δ_(Ref/Sub) detector 462 detects δ_(Ref/Sub) from signals I_(ref) andI_(het).δ_(Ref/Sub)=(φ_(Ref)+φ_(noise1))−(φ_(het)+φ_(Sub)+φ_(noise2))  (6)

-   -   Where, δ_(Ref/Sub) is a reference phase shift due to the        substrate,    -   φ_(Ref) is the phase shift associated with the reference        detector from BS 412,    -   φ_(noise1) is the phase shift associated with the detector        noise,    -   φ_(het) is the phase shift associated with the heterodyne        measurement detector from BS 412,    -   φ_(noise2) is the phase shift associated with the detector        noise, and    -   φ_(Sub) is the phase shift associated with the substrate.

Notice that unlike Equation (5), Equation (6), for finding δ_(Ref/Sub),does not contain any terms that depend on the film phase shift, andtherefore, the value of δ_(Ref/Sub) is unaffected by changes in the filmphase (i.e., changes in the thickness of the film).

FIG. 5 is a flowchart depicting the method for finding a temperatureindependent film thickness using a self-referencing heterodynereflectometry in accordance with an exemplary embodiment of the presentinvention. The process begins by propagating HR beam with s-polarizationat frequency ω and p-polarization at frequency ω+Δω to a target sampleat α angle of incidence (step 502). The reflected HR beam is detected ata reference detector and a heterodyne measurement detector (step 504)and δ_(Ref/film) determined from I_(ref) and I_(het) signals from therespective detectors (step 506). The self-referencing mode processing issimilar. An SR beam with p-polarization at frequency ω andp-polarization at frequency ω+Δω is propagated to a target sample at aangle of incidence (step 508). The reflected SR beam is detected at areference detector and a heterodyne measurement detector (step 510) andδ_(Ref/Sub) determined from I_(ref) and I_(het) signals from therespective detectors (step 512). Next, temperature independent phaseshift, Δφ_(film), attributable to the film is calculated from differenceof δ_(Ref/Sub) and δ_(Ref/film) (step 514). Finally, the film thickness,d_(f), can be calculated from Δφ, n_(f), α and λ using, for example,Equation (4) (step 516). The refractive index, n_(f), for the particularfilm should be known beforehand. Alternatively, the measurement detectorof the self-referencing heterodyne reflectometer can be augmented with agrating interferometer as disclosed in U.S. patent application Ser. Nos.11/066,933 and 11/178,856, from which the refractive index, n_(f), forthe film and be dynamically measured.2Δφ_(film)=δ_(Ref/Sub)−δ_(Ref/film)  (7)

Where, Δφ_(film) is the phase shift due to the film layer.

Using Equation (7), the phase shift due to the film layer, Δφ_(film), iscalculated by Δφ_(film) calculator 466 subsequent to each δ_(Ref/Sub)and δ_(Ref/film) measurement. Assuming the noise levels betweensuccessive measurements are the same (or sufficiently small), thethickness, d_(f), of film 214 can then be determined directly by d_(f)calculator 468 using Equation (4) above, with the refractive index forthe particular film, n_(f), the wavelength, λ, of light source 400, andthe angle of incidence, α.

The level of detector noise may be monitored by comparing successiveδ_(Ref/Sub) measurements for changes. Recall that δ_(Ref/Sub) iscalculated from a self referencing beam that is unaffected by changes infilm thickness, hence δ_(Ref/Sub) is also unaffected by changes in filmthickness. From Equation (6) above, it is apparent that the value ofδ_(Ref/Sub) will not change between successive δ_(Ref/Sub) measurementsunless the level of detector noise changes. Therefore, the severity ofthe detector noise can be determined by comparing the change insuccessive δ_(Ref/Sub) measurements to a noise threshold.

Therefore, in accordance with another aspect of the present invention,detector noise is monitored and when the noise level is unacceptable,the Δφ_(film) is averaged over several measurement cycles. Returning toFIGS. 4A-4C, threshold noise detector 465 monitors successiveδ_(Ref/Sub) measurements from δ_(Ref/Sub) detector 462 and compareschanges in the noise level to a threshold. If the level is below thethreshold level, threshold noise detector 465 takes no action, but ifthe noise level is found to be higher than the acceptable noisethreshold, δ_(Ref/Sub) detector 462 instructs Δφ_(film) averager 467 toaverage several or more cycles of Δφ_(film) data from Δφ_(film)calculator 466, and output an averaged Δφ_(film(AVG)) to d_(f)calculator 468.

FIG. 5 is a flowchart depicting the method for finding a temperatureindependent film thickness using a self-referencing heterodynereflectometry in accordance with an exemplary embodiment of the presentinvention. The process begins by propagating HR beam with s-polarizationat frequency ω and p-polarization at frequency ω+Δω to a target sampleat a angle of incidence (step 502). The reflected HR beam is detected ata reference detector and a heterodyne measurement detector (step 504)and δ_(Ref/film) determined from I_(ref) and I_(het) signals from therespective detectors (step 506). The self-referencing mode processing issimilar. An SR beam with p-polarization at frequency ω andp-polarization at frequency ω+Δω is propagated to a target sample at αangle of incidence (step 508). The reflected SR beam is detected at areference detector and a heterodyne measurement detector (step 510) andδ_(Ref/Sub) determined from I_(ref) and I_(het) signals from therespective detectors (step 512). Next, temperature independent phaseshift, Δφ_(film), attributable to the film is calculated from differenceof δ_(Ref/Sub) and δ_(Ref/film) (step 514). Finally, the film thickness,d_(f), can be calculated from Δφ, n_(f), α and λ using, for example,Equation (4) (step 516). The refractive index, n_(f), for the particularfilm should be known beforehand. Alternatively, the measurement detectorof the self-referencing heterodyne reflectometer can be augmented with agrating interferometer as disclosed in U.S. patent application Ser. Nos.11/066,933 and 11/066,933, from which the refractive index, n_(f), forthe film and be dynamically measured.

FIGS. 6A and 6B diagrammatically illustrate the interactions between theHR beam and/or SR beam with the film and substrate. FIG. 6A shows the HRbeam interactions and FIG. 6B depicts the SR beam interactions. FIG. 6Ais essentially a composite of FIGS. 3A and 3B discussed above, and showsincident HR beam 403 comprised of two linearly polarized s- andp-polarized beam components that are orthogonal to each other, withsplit optical frequencies of ω and ω+Δω. The s-polarization componentinteracts with the surface of film 214 and is partially reflected as ray405-1. Ray 405-1 is almost completely s-polarization. Incident angle αis at the Brewster of film 214 for optimizing the reflecteds-polarization component of ray 405-1. On the other hand, thep-polarization component does not interact with the surface of film 214and refracts from the interface between film 214 and substrate 212 atangle ρ as ray 405-2. However, because some of the s-polarizationcomponent is also refracted, ray 405-2 comprises both s- andp-polarization components. Clearly, as the thickness of film 214changes, the distance traversed by the HR beam (beam 403 and ray 405-1will change and consequently the phase will also experience acorresponding change at the detector.

FIG. 6B shows incident SR beam 433 comprised of two linearly polarizedp- and p-polarized beam components, with split optical frequencies of ωand ω+Δω. With incident angle α approximating the Brewster of film 214,only a minimal reflection of the p-polarized SR beam occurs at thesurface of film 214. Instead, incident SR beam 433 refracts into film214 and reflects off the interface between film 214 and substrate 212 atangle ρ as ray 435. Unlike the HR beam, the SR beam is unaffected bychanges in the thickness of film 214 because the beam does not interactwith the surface of the film. The beat signals generated by the SR beamscan be used to obtain a reference phase value, which is equivalent tothat obtained using a reference sample. This reduces the need to haveperiodic access to reference wafer. The availability of a referencephase, which is unaffected by changes in film thickness, but drifts withtemperature by a corresponding amount as the measured film phase, allowsfor real-time phase drift corrections to the measured film phase.

In addition to compensating for temperature related phase drift andeliminating the necessity for calibration wafers, the availability of areference phase also provides a mechanism for assessing detector noise.As mentioned above, temperature induced phase drift (or noise) from thedetector can be assumed to be the same as for successive measurementsand therefore can be canceled out. However, it is possible for the levelof spurious noise in the detector to reach a level where this may nothold true. In that case, merely canceling the noise may provide aninferior result. In accordance with another exemplary embodiment of thepresent invention, the level of detector noise can be monitored inreal-time, thereby providing a basis for implementing more rigorousnoise reduction measures.

FIG. 7 is a flowchart depicting the method for identifying detectornoise that may be resistive to the noise canceling in accordance with anexemplary embodiment of the present invention. The process begins byfinding successive values of δ_(Ref/Sub) from successive SR beammeasurements, i.e., δ_(Ref/Sub1) and δ_(Ref/Sub2) (step 702). Recallthat reference phase δ_(Ref/Sub) is unaffected by changes in filmthickness, but is affected by detector noise and drift with temperature.Phase drift, and other noise affecting the phase, is assumed to benegligible between successive measurements made in rapid succession.However, some noise may exist. If δ_(Ref/Sub1)−δ_(Ref/Sub2)=0, thedetector noise and/or drift are negligible. However, where the phasedifference between successive measurements is greater than 0, i.e.,δ_(Ref/Sub1)−δ_(Ref/Sub2)>0, some noise is present and, depending on theamount, should be suppressed. A noise threshold level can be adopted fora particular application, below which the results are acceptable andadditional suppression is not necessary. Therefore, threshold noisedetector 465 compares the phase difference between successive referencephase measurements to a noise threshold, i.e.,|δ_(Ref/Sub1)−δ_(Ref/Sub2)|>THRESHOLD (step 704). If the noise incrementis below the threshold level, d_(f) calculations proceed by finding thefilm induced phase shift, Δφ_(film), from the measurement phaseδ_(Ref/film) and the reference δ_(Ref/Sub) (step 706) and then the filmthickness d_(f) from Δφ_(film), n_(f), α and λ (step 710). If, at step704, |δ_(Ref/Sub1)−δ_(Ref/Sub2)| is greater than the noise threshold,additional noise suppression procedures should be implemented. Oneexemplary procedure is to smooth the noise profile by averaging theresult over several successive measurement cycles (step 708). Any ofΔφ_(film), δ_(Ref/Sub) and δ_(Ref/film) or d_(f) can be averaged, butaveraging Δφ_(film) or δ_(Ref/Sub) and δ_(Ref/film) can be accomplishedin earlier stages of the process. In any case, film thickness d_(f) fromΔφ_(film), n_(f), α and λ, albeit and averaged thickness (step 710).

The present invention, as depicted in FIGS. 4A-4C, rapidly alternatesbetween HR mode and SR modes for detecting δ_(Ref/film) phasemeasurement and δ_(Ref/Sub) reference phase by slidingpolarizer/λ/2—aperture component in the path of the HR beam. FIGS. 8Aand 8B are diagrams of a self referencing heterodyne reflectometerconfigured without moving optical components in accordance with anexemplary embodiment of the present invention. FIG. 8A shows the selfreferencing heterodyne reflectometer in HR mode for detecting aδ_(Ref/film) phase measurement and FIG. 8B depicts the reflectometer inSR mode for detecting δ_(Ref/Sub) reference phase. Much of the structureis similar to that discussed above with regard to FIGS. 4A-4C, andtherefore only the distinctions will be discussed in greaterspecificity.

In accordance with this exemplary embodiment, HR beam 802 is selectivelypropagated in an HR path and an SR path. Sliding shutter 809 selectivelyopens one path, while simultaneously closing the other. Slidercontroller 461 provides the operational control signals forrepositioning sliding shutter 809. In the HR mode the HR path is openwith sliding shutter 809 blocking the SR path. HR beam 802 reflects offBS 812 as beam 804, through polarizer 815, to detector 816, resulting inreference signal I_(ref). Incident HR beam 803, the portion of HR beam802 transmitted through BS 812, and across optional reflective opticalcomponents 818 and 820, interacts with film 214, and on to detector 826,via polarizer 822 and focusing optics 824, as rays 805-1 and 805-2.Slider controller 461 receives signals I_(ref) and I_(het) as describedabove, which are passed to δ_(Ref/film) detector 463 for detection ofδ_(Ref/film) measurement phase.

In the SR mode, sliding shutter 809 blocks the HR path and opens the SRpath. HR beam 802 from light source 800, is deflected at BS 801 andreflected at optical component 828 to stationary polarizer/λ/2combination 810/811 where SR beam 833 is formed. Recall that HR beam isa split frequency, linearly polarized where one polarization componentat frequency ω is orthogonal with respect to the other polarizationcomponent at frequency ω+Δω. The SR beam is a split frequency,p-polarized beam. Incident SR beam 833 converges back to the path of theincident HR beam 803 at BS 807. SR beam 833 reflects off BS 812 as beam834 to detector 816, resulting in reference signal I_(ref). The portionof SR beam 833 transmitted through BS 812, interacts with film 214, andon to detector 826 as ray 835. Slider controller 461 receives signalsI_(ref) and I_(het) as described above, which are passed to δ_(Ref/Sub)detector 462 for detection of δ_(Ref/Sub) measurement phase.

In this exemplary embodiment, a significant amount of light is lost andhence light source 800 should be selected to accommodate the loss oflight. As a sidebar, the combination of beam splitters 801 and 807 withreflection components 828 and 829 suggest the look of a Mach Zehnderinterferometer, but because self referencing beam 833 and the HR beam803 are not used simultaneously, there is no optical interferencebetween them and hence no finite fringe issue. Also, the different pathstraveled by the beams before reaching BS 812 will have no effect onphase measurement since the phase differencing, between I_(Ref) andI_(het) signals, for each beam is accomplished after BS 812.

The δ_(Ref/Sub) reference phase provides a reference from which accuratetemperature independent film phase shifts, Δφ_(film), may be derivedwithout the use of a reference wafer. It can be assumed thatφ_(Sub1)≈φ_(Sub2)≈φ_(Subn) across a wafer, and therefore the HR and SRbeam spots on a film need not be coextensive. Thus, the self referencingheterodyne reflectometer depicted in FIGS. 8A and 8B can be madesignificantly less lossy by separating the SR beam and HR beam paths.

FIGS. 9A and 9B are diagrams of a self referencing heterodynereflectometer with separate SR beam and HR beam paths in accordance withan exemplary embodiment of the present invention. The self referencingheterodyne reflectometer shown in FIGS. 9A and 9B is identical to thatdepicted in FIGS. 8A and 8B with the exception of the paths of theincident and reflected SR beams. Rather than utilizing a pair of beamsplitters for diverging and collimating the separate beam paths, SR beam933 propagates in an essentially parallel path to that of HR beam 903via reflection optics 917 which replaces BS 807, is out of line with BS901. HR beam 902 is deflected at BS 901 into the SR path to stationarypolarizer/λ/2 combination 910/911 where SR beam 933 is formed, and on toreflection optics 917.

FIGS. 10A and 10B are diagrams of a self referencing heterodynereflectometer with counter rotating SR and HR beam paths in accordancewith an exemplary embodiment of the present invention. Two synchronousshutters are necessary for switching modes, one at either detector. Asshould be appreciated, because the HR and SR beams propagate indirections counter to each other, detector 1016 detects signal I_(ref)for HR beam 1004 and I_(het) for SR beam 1035. Conversely, detector 1026detects signal I_(ref) for SR beam 1034 and I_(het) for HR beam 1005. InHR mode, light source 1000 generates HR beam 1002 beam is reflected atBS 1018 as incident HR beam 1003 and off BS 1041, passed open shutter1051, through focusing optics 1015, to detector 1016. At BS 1041, thetransmitted portion of beam 1003 propagates in a counter clockwisedirection (with respect to FIGS. 10A and 10B) off optical reflector 1020to film 214 and reflected HR beam 1005 continues off optical reflector1021, through BS 1023 and passed open shutter 1052, through polarizer1022 and focusing optics 1024, to detector 1026. In SR mode, HR beam1002 beam is transmitted through BS 1018 to polarizer/λ/2 1010/1011 andconverted to p-polarized, split frequency SR beam 1033. At BS 1023 thetransmitted portion of SR beam 1033 is turned at corner cube 1050 to BS1023 and passed open shutter 1052 to detector 1026. At BS 1023 thereflected portion of SR beam 1033 propagates in a direction counter toHR beam 1003 off optical reflector 1021 to film 214 and reflected SRbeam 1035 continues off optical reflector 1020, reflecting off throughBS 1042 and passed open shutter 1051 to detector 1016.

FIGS. 11A and 11B are diagrams of a self referencing heterodynereflectometer employing a liquid crystal variable retarder (LCVR) forelectronically switching between HR and SR operating modes in accordancewith an exemplary embodiment of the present invention. The configurationof self referencing heterodyne reflectometer shown in FIGS. 11A and 11Bis similar to that depicted in FIGS. 8A and 8B with the exception ofpolarizing BS 1119 in the HR beam path and LCVR 1111 in the SR beam pathbut operationally is quite different. LCVR 1111 is a device, whichcauses the polarization of a light beam to be rotated by an angle, whichis dependent upon the voltage applied to it. When the retarder is set sothat the polarization is not rotated, the device operates as aheterodyne interferometer as previously described. When the retarder isset to rotate the polarization by 90°, then the beams at bothfrequencies are p-polarized, and the SR function is obtained. In thisembodiment, the amount of light lost is reduced. On the other hand, thepath between PBS 1119 and BS 1107 acts as a Mach Zehnder interferometer.

In the HR mode, HR beam 1102 is separated into p- and s-polarizationcomponents at polarizing beam splitter PBS 1119, the p-polarizationcomponent (at frequency ω+Δω) propagates as beam 1103 and thes-polarization component (at frequency ω) propagates as beam 1133. Beam1103 reflects off BS 1112 as beam 1104 to detector 1116. Incident HRbeam 1103, the portion of HR beam 1102 transmitted through BS 1112,interacts with film 214, and on to detector 1126 as rays 1105. Thes-polarization component of HR beam 1102 passes through LCVR 1111, whichis switched OFF in HR mode, as beam 1133. Beam 1133 reflects off BS 1107and also reflects off BS 1112 as beam 1134 to detector 1116. HR beams1134 and 1104 result in reference signal I_(ref). The portion of beam1133 transmitted through BS 1112 interacts with film 214, and on todetector 1126 as rays 1135-1 and 1135-2. Taken together, beams 1103 and1133 are HR. Reflected HR beam components 1105, 1135-1 and 1135-2 resultin heterodyne measurement signal I_(het).

In the SR mode, HR beam 1102 is separated into p- and s-polarizationcomponents at polarizing beam splitter PBS 1119, with the p-polarizationcomponent (at frequency ω+Δω) propagating as beam 1103 as describedabove. The s-polarization component of HR beam 1102 is transformed intop-polarized beam 1133 by LCVR 1111, which is ON in SR mode. Beam 1133reflects off BS 1107 and again off BS 1112 as beam 1134 to detector1116. Beams 1134 and 1104 result in reference signal I_(ref) for the SRmode. The portion of beam 1133 transmitted through BS 1112 interactswith film 214, and on to detector 1126 as ray 1135. Reflected SR beamcomponents 1105 and 1135 result in heterodyne measurement signalI_(het).

FIGS. 12A and 12B are diagrams of a self referencing heterodynereflectometer in which the SR beam bypasses the sample for addressingthe sole issue of detector phase drift in accordance with an exemplaryembodiment of the present invention. To remove the effects of phasedrift from the system, the split frequency light is measured in analternating fashion by the detector 1226. In the HR mode, HR beam 1202is reflected as HR beam 1203 by beam splitter 1218. Beam 1203 reflectsoff BS 1212 as beam 1204 to detector 1216, resulting in reference signalI_(ref) for the HR mode. Incident HR beam 1203 interacts with film 214,passing open shutter 1217 and on to detector 1226 as rays 1205 (actually1205-1 and 1205-2), resulting in heterodyne measurement signal I_(het)for the HR mode. The portion of beam 1202 transmitted through BS 1218 isblocked by shutter 1209. In wafer measurement mode (SR mode) shutters1209 and 1217 reverse their positions with shutter 1217 blockingreflected HR beam components 1205, and shutter 1209 in the openposition. Beam 1205 transmitted through BS 1218 is measured at detector1226, while reflected beam 1204 is measured at detector 1216. Thismeasurement allows the ability to determine the phase offset betweeneach detector just prior to or after each measurement of the wafer.

The present invention, is directed to a self-referencing heterodynereflectometer which rapidly alternates between HR mode and SR modes fordetecting a δ_(Ref/film) phase measurement and a δ_(Ref/Sub) referencephase. The exemplary embodiment depicted in FIGS. 4A-4C utilizes asliding polarizer/λ/2—aperture component in the path of the HR beam tocreate the SR beam. In accordance with another embodiment of the presentinvention, a self-referencing heterodyne reflectometer is disclosed thatalternates between HR mode and SR modes by propagating the HR beam in aseparate path from the SR beam, thereby allowing the HR and SR beams tobe separately controlled. In accordance with the exemplary embodimentdisclosed in FIGS. 8A and 8B, a sliding shutter, under the control of aslider controller, is employed to alternate the beams on the target,between measuring δ_(Ref/film) phase measurement with the HR beam andmeasuring δ_(Ref/Sub) with the SR beam (of course controller 461 alsoroutes the I_(het) and I_(ref) signals to either of δ_(Ref/film) phasedetector 463 or δ_(Ref/Sub) phase detector 462 depending on which beamis incident on the target). As a practical matter, however, slidingshutters are somewhat slow which results in a relatively long cycle timefor calculating of Δφ_(film) data. As mentioned elsewhere above, errorin the Δφ_(m) measurement is the detector error due to the effect oftemperature on the detectors. Thus, the detector temperature error isgreater with a longer measurement cycle and, consequently, can bereduced by shortening the measurement cycle.

Therefore, in accordance with another exemplary embodiment of thepresent invention, a high frequency optical switch is employed forrapidly alternating between the HR beam and the SR beam in themeasurement cycle. One such optical switch is a rotating chopper.Rotating optical choppers are well known in the related art as a metaldisc with slots etched into it and is mounted on a drive axle androtated. The disc is placed in the beam path which causes the beam to beperiodically interrupted by the blocking part of the disc. Thus, themeasurement beam can rapidly switch from HR mode to SR mode, and viceversa, thereby greatly reducing the time period for the detectortemperature to drift and thereby prevent unwanted temperature inducederror in the phase measurements. It should be understood that althoughthe embodiments of the present invention below are described withreference to a rotating optical chopper, the chopper is merely anexemplary device for switching the heterodyne reflectometer between HRmode and SR modes at a higher rate than temperature change in thedetector. In so doing, any error in the phase measurement due todetector temperature will be comparable in consecutive HR and SRmeasurements and effectively cancel out in the phase calculations. Thoseof ordinary skill in the art will readily understand that other opticalswitching devices may exist and/or will exist in the future that areequivalent to a mechanical chopper for the purposes discussedhereinabove.

In this regard, FIG. 13A and FIG. 13B show an exemplary self referencingheterodyne reflectometer which employ a mechanical orelectrical-electromagnetic device for switching between the HR and SRbeams, or vice versa, at a faster rate than the temperature drift in adetector. The figures illustrate the operational modes of an exemplaryself referencing heterodyne reflectometer which is similar to thatdiscussed above with respect to FIGS. 8A and 8B. FIG. 13A shows the selfreferencing heterodyne reflectometer in HR mode for detecting aδ_(Ref/film) phase measurement and FIG. 13B depicts the reflectometer inSR mode for detecting δ_(Ref/Sub) reference phase. Much of the structureis similar to that discussed above with regard to FIGS. 4A-4C, as wellas FIGS. 8A and 8B and therefore only the distinctions will be discussedin greater specificity.

In accordance with this exemplary embodiment, HR beam 1302 isselectively propagated in an HR path and an SR path. High frequencyoptical switch 1309 selectively opens one path, while simultaneouslyclosing the other. Optical switch 1309 is depicted in the figure as apair of rotating optical choppers, one positioned in the path of HR beam1303 and the other positioned in the path of SR beam 1333. Opticalswitches 1309 are out of phase such that as the path closes, completingthe first part of the measurement cycle, the opposite paths opens forcompletion of the entire measurement cycle. Alternatively, a singleoptical switch, such as a rotating chopper, could be positioned acrossboth beam paths, with the opening slots out of phase with respect to thebeams. As discussed with regard to FIGS. 8A and 8B above, choppercontroller 1361 provides the operational control signals forrepositioning (rotating) optical choppers 1309 and synchronizes them tothe measurement cycle if the self referencing heterodyne reflectometeris configured with a pair of choppers rather than a single chopper. Inthe HR mode, depicted in FIG. 13A, the HR path is open with chopper 1309blocking the SR path. HR beam 1303 reflects off BS 1312 as beam 1304 todetector 1316, resulting in reference signal I_(ref). Incident HR beam1303, the portion of HR beam 1303 transmitted through BS 1312, interactswith film 214, and on to detector 1326 as rays 1305-1 and 1305-2.Chopper controller 1361 receives signals I_(ref) and I_(het) asdescribed above, which are passed to δ_(Ref/film) detector 463 fordetection of δ_(Ref/film) measurement phase.

In the SR mode, depicted in FIG. 13B, chopper 1309 blocks the HR pathand in so doing aligns an opening slit with the SR path, thereby openingthe SR path. HR beam 1302 from light source 1300, is deflected at BS1301 and reflected at optical component 1328 to stationary polarizer/λ/2combination 1310/1311 where SR beam 1333 is formed. Recall that HR beamis a split frequency, linearly polarized where one polarizationcomponent at frequency ω is orthogonal with respect to the otherpolarization component at frequency ω+Δω. The SR beam is a splitfrequency, p-polarized beam. Incident SR beam 1333 converges back to thepath of the incident HR beam 1303 at BS 1307. SR beam 1333 reflects offBS 1312 as beam 1334 to detector 1316, resulting in reference signalI_(ref). The portion of SR beam 1333 transmitted through BS 1312,interacts with film 214, and on to detector 1326 as ray 1335. Choppercontroller 1361 receives signals I_(ref) and I_(het) as described above,which are passed to δ_(Ref/Sub) detector 462 for detection ofδ_(Ref/Sub) measurement phase.

In this exemplary embodiment, a significant amount of light is lost andhence light source 1300 should be selected to accommodate the loss oflight. As a sidebar, the combination of beam splitters 1301 and 1307with reflection components 1328 and 1329 suggest the look of a MachZehnder interferometer, but because self referencing beam 1333 and theHR beam 1303 are not used simultaneously, there is no opticalinterference between them and hence no finite fringe issue. Also, thedifferent paths traveled by the beams before reaching BS 1312 will haveno effect on phase measurement since the phase differencing, betweenI_(Ref) and I_(het) signals, for each beam is accomplished after BS1312.

The δ_(Ref/Sub) reference phase provides a reference from which accuratetemperature independent film phase shifts, Δφ_(film), may be derivedwithout the use of a reference wafer. It can be assumed thatφ_(Sub1)≈φ_(Sub2)≈φ_(Subn) across a wafer, and therefore the HR and SRbeam spots on a film need not be coextensive. Thus, the self referencingheterodyne reflectometer depicted in FIGS. 13A and 13B can be madesignificantly less lossy by separating the SR beam and HR beam paths.

Although not depicted in a figure, the self referencing heterodynereflectometer shown in FIGS. 9A and 9B may also employ a high frequencyoptical switch, such as a rotating chopper, for generating parallel pathHR and SR beams.

FIGS. 14A and 14B are diagrams of a self referencing heterodynereflectometer with counter rotating SR and HR beam paths which utilize ahigh frequency optical switch to minimize error in the phase measurementresulting from changes in the temperature of the detectors in accordancewith an exemplary embodiment of the present invention. The selfreferencing heterodyne reflectometer depicted in FIGS. 14A and 14B isidentical to that shown in FIGS. 10A and 10B, with the exception of theoptical switches. Two synchronous choppers are necessary for rapidlyswitching modes, one at either detector, as shown both choppers have twobeam channels, one for regulating the HR path and the other for the SRpath (in so doing the beams can be incident the respective chopper at anidentical azimuth but in different optical channels). As discussed abovewith regard to FIGS. 10A and 10B, the HR and SR beams propagate indirections counter to each other and, therefore, detector 1416 detectssignal I_(ref) for HR beam 1404 and I_(het) for SR beam 1435. Detector1426 detects signal I_(ref) for SR beam 1434 and I_(het) for HR beam1405. In HR mode, choppers 1451 and 1452 rotate open with respect to HRbeam 1404 and HR beam 1405, respectively, thereby allowing the beams topropagate incident to detector 1416 and detector 1426, respectively.Choppers 1451 and 1452 simultaneously rotate closed with respect to SRbeam 1435 and SR beam 1434. At the close to the HR mode, the HR portionof the measurement cycle, choppers 1451 and 1452 rotate closed withrespect to HR beam 1404 and HR beam 1405 and open to SR beam 1435 and SRbeam 1434. SR beam 1435 and SR beam 1434 propagate incident to detector1416 and detector 1426, which produce I_(het) and I_(ref) signals, butat the opposite detectors from the HR mode.

FIGS. 15A and 15B are diagrams of a self referencing heterodynereflectometer in which the SR beam bypasses the sample for addressingthe sole issue of detector phase drift in accordance with an exemplaryembodiment of the present invention. Although the use of high frequencyoptical switches reduce the amount of error resulting from temperaturerelated detector drift, some phase drift is inevitable, the effects ofwhich can be measured as discussed above with regard to FIGS. 12A and12B.

Although the self referencing heterodyne reflectometer embodimentsdiscussed above are highly accurate and stable, they suffer from twoshortcomings. First, in an effort to reduce the disparity in themeasurement between the SR beam and the HR beam, each of the previousembodiments use a single light source for generating the SR and HRbeams. Thus, the strength of the beam is reduced by half for eitheroperation mode (with the exception of those using a shutter which sufferfrom other shortcomings discussed immediately above). Furthermore, andsecondly, the use of parallel SR and HR beam paths increases thecomplexity of the setup and alignment. These and other shortcoming areovercome by the use of a second light source in conjunction with anamplitude modulator, for generating an amplitude modulated (AM)reference beam. Modulating the amplitude of the independently generatedlight beam between the two modulated amplitudes of α and α+Δα results inreference and heterodyne signals that allow for accurate phasedetections regardless of the path distances to the separate detector,similar to that discussed above for using HR beam components atfrequencies ω and ω+Δω.

FIGS. 16A and 16B are diagrams of a self referencing heterodynereflectometer which utilizes an amplitude modulated beam as a referencein conjunction with the HR beam in accordance with an exemplaryembodiment of the present invention. The self referencing heterodynereflectometer described herein employs a chopper 1609 for switchingbetween HR and amplitude modulated (AM) operating modes in accordancewith an exemplary embodiment of the present invention. The configurationof the HR path in the self referencing heterodyne reflectometer shown inFIGS. 16A and 16B is similar to that depicted in FIGS. 13A and 13B,however the SR beam path is replaced by an amplitude modulated (AM)beam. In accordance with this exemplary embodiment, light source 1600generates HR beam 1602 having two linearly polarized components,operating at split optical angular frequencies, that are orthogonal withrespect to each other for illuminating the target; an s-polarized beamcomponent at frequency ω and a p-polarized beam component at frequencyω+Δω. HR beam 1602 is propagated in only an HR path. In furtheraccordance with this exemplary embodiment, light source 1611 generates abeam that's amplitude is modulated by amplitude modulator 1613,resulting is AM beam 1601 having a single frequency, ω′, at twomodulated amplitudes of α and α+Δα. The frequency, ω′ may be differentfrom the frequency of the HR beam, ω, however, amplitude modulator 1613oscillates the amplitudes operates at approximately the frequency of theHR beam, ω. Furthermore, although the AM beam is depicted as ap-polarized beam, it need only have a p-component.

Chopper 1609 selectively opens one path, while simultaneously closingthe other. A chopper controller (not shown) provides the operationalcontrol signals for the chopper 1609 and detector signal paths (seediscussion of FIGS. 4 and 8). In the HR mode (depicted in FIG. 16A), theHR path is open with chopper 1609 blocking the AM beam path. HR beam1602 passes through BS 1612 as beam 1604 to detector 1616, resulting inreference signal I_(ref). HR beam 1603, the portion of HR beam 1602reflecting off BS 1612, interacts with film 214, and on to detector 1626as rays 1605-1 and 1605-2 resulting in reference signal I_(ref).δ_(Ref/film) phase measurement is detected from signals I_(ref) andI_(het) of the HR beam in the same manner as described above.

In the AM mode (depicted in FIG. 16B), chopper 1609 blocks the HR pathand opens the AM path. The amplitude of the beam from light source 1601,is modulated at amplitude modulator 1613 and a portion deflected at BS1612 as beam 1634 to detector 1616, resulting in reference signalI_(ref). The portion of AM beam 1603 that passes through BS 1612,interacts with film 214, and on to detector 1626 as ray 1635, resultingin signal I_(het). δ_(Ref/Sub) phase measurement is detected fromsignals I_(ref) and I_(het) of the AM beam in the same manner asdescribed above with respect to the SR beam. Phase φ_(film) and the filmthickness, d_(f), are derived from δ_(Ref/film) phase and δ_(Ref/Sub)phase as also discussed above.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A method for measuring a thickness parameter comprising: measuring aheterodyne phase shift, comprising: providing a split frequency, dualpolarized beam; detecting a reference signal from the split frequency,dual polarized beam; propagating the split frequency, dual polarizedbeam to a target; receiving a reflected split frequency, dual polarizedbeam from the target; detecting a measurement signal from the reflectedsplit frequency, dual polarized beam; and measuring a phase differencebetween the reference signal and the measurement signal for thereflected split frequency, dual polarized beam; measuring a selfreferencing phase shift, comprising: providing a single frequency,amplitude modulated beam; detecting a second reference signal from thesingle frequency, amplitude modulated beam; propagating the singlefrequency, amplitude modulated beam to the target; receiving a reflectedsingle frequency, amplitude modulated beam from the target; detecting asecond measurement signal from the reflected single frequency, amplitudemodulated beam; and measuring a self referencing phase differencebetween the second reference signal and the second measurement signalfor the amplitude modulated beam; finding a phase difference for thetarget from the heterodyne phase shift and the self referencing phaseshift; and calculating a thickness parameter for the target from thephase difference for the target.
 2. A self referencing heterodynereflectometer, comprising: a heterodyne reflectometry beam source forgenerating a heterodyne reflectometry beam; a self referencing amplitudemodulated beam source for generating a self referencing amplitudemodulated beam; an operating mode switcher for receiving at least one ofthe heterodyne reflectometry beam and the self referencing amplitudemodulated beam and selectively outputting one of the heterodynereflectometry beam and the self referencing amplitude modulated-beam; areference detector for receiving the heterodyne reflectometry beam andgenerating a reference heterodyne reflectometry phase signal, and forreceiving the self referencing amplitude modulated beam and generating areference self referencing phase signal; a target material; firstoptical elements for propagating the heterodyne reflectometry beam andthe self referencing beam incident to the target material at apredetermined angle of incidence; a measurement detector for receiving areflected heterodyne reflectometry beam from the target material andgenerating a measurement heterodyne reflectometry phase signal, and forreceiving a reflected self referencing amplitude modulated beam from thetarget material and generating a measurement self referencing amplitudemodulated phase signal; a heterodyne phase shift detector for detectinga heterodyne phase shift between the reference heterodyne reflectometryphase signal and the measurement heterodyne reflectometry phase signal;a self referencing phase shift detector for detecting a reference phaseshift between the reference self referencing amplitude modulated phasesignal and the measurement self referencing phase signal, and athickness calculator for calculating a thickness parameter for thetarget material from the heterodyne phase shift and the reference phaseshift.
 3. The self referencing heterodyne reflectometer recited in claim2, further comprising: a phase shift calculator for receiving theheterodyne phase shift and the reference phase shift and for calculatinga phase shift induced by the target material.
 4. The self referencingheterodyne reflectometer recited in claim 2, wherein the heterodynereflectometry beam source produces a split frequency, dual polarizedbeam and the self referencing amplitude modulated beam source produces asingle frequency, dual amplitude beam having at least a p-polarizationbeam component.
 5. The self referencing heterodyne reflectometer recitedin claim 2, wherein the operating mode switcher is an optical chopper.6. The self referencing heterodyne reflectometer recited in claim 2,wherein one of the first optical elements and the second first opticalelements is comprised of material based on thermal stress inducedbirefringence properties.
 7. The self referencing heterodynereflectometer recited in claim 6, wherein the material is one of a fusedsilica, BK7 silica and SF57 silica.
 8. The self referencing heterodynereflectometer recited in claim 2, wherein the heterodyne reflectometrybeam source generates an s-polarized beam component at the firstfrequency and a p-polarized beam component at the second frequency. 9.The self referencing heterodyne reflectometer recited in claim 8,wherein the operating mode switcher propagates one of the heterodynereflectometry beam and the self referencing amplitude modulated beamwhile simultaneously obstructs the other of the heterodyne reflectometrybeam and the self referencing amplitude modulated beam, therebyswitching a measurement beam at the measurement detector and betweenheterodyne reflectometry operating mode and self reference operatingmode.
 10. The self referencing heterodyne reflectometer recited in claim9, wherein the thickness parameter is a thickness of a layer in thetarget material.
 11. The self referencing heterodyne reflectometerrecited in claim 10, further comprises: a threshold noise detector formonitoring a reference phase measurement from successive self referenceoperating modes and comparing changes between successive reference phasemeasurements to a predetermined threshold noise level.
 12. The selfreferencing heterodyne reflectometer recited in claim 9, furthercomprises: a chopper controller for iterating between heterodynereflectometry operating mode and self reference operating mode, saidchopper controller being operationally coupled to the operating modeswitcher and between the measurement detector and both the heterodynephase shift detector and the self referencing phase shift detector andbetween the reference detector both the heterodyne phase shift detectorand the self referencing phase shift detector, wherein in the selfreferencing operating mode, the operating mode switcher obstructs a pathof the heterodyne reflectometry beam and said chopper controllerinstructs the self referencing phase shift detector to detect thereference phase shift between the reference self referencing phasesignal and the measurement self referencing amplitude modulated phasesignal, and wherein in heterodyne reflectometry operating mode, theoperating mode switcher obstructs a path of the self referencingamplitude modulated beam and said chopper controller instructs theheterodyne phase shift detector to detect the heterodyne phase shiftbetween the reference heterodyne reflectometry phase signal and themeasurement heterodyne reflectometry phase signal.
 13. The selfreferencing heterodyne reflectometer recited in claim 9, furthercomprises: a phase shift averager for averaging successive phase shiftsfrom the phase shift calculator based on a noise determination of one ofthe reference detector and the measurement detector.
 14. The selfreferencing heterodyne reflectometer recited in claim 4, wherein afrequency of modulation for the dual amplitudes by the self referencingamplitude modulated beam source is based on one or both of the splitfrequencies of the heterodyne reflectometry beam source.
 15. Theheterodyne reflectometer recited in claim 2, wherein the predeterminedangle of incidence is related to a refractive index for the targetmaterial.
 16. The heterodyne reflectometer recited in claim 2, whereinthe predetermined angle of incidence is a predetermined default angle.17. The heterodyne reflectometer recited in claim 2, wherein thepredetermined angle of incidence approximates Brewster's angle for thetarget material.
 18. A heterodyne reflectometer for measuring thicknessparameter of a target layer deposed on a substrate comprising: a opticalchopper for receiving a heterodyne reflectometry beam and a selfreferencing beam and obstructing one of the heterodyne reflectometrybeam and the self referencing beam; a first heterodyne detector forreceiving the heterodyne reflectometry beam and generating a referenceelectrical heterodyne beat signal and for receiving the self referencingbeam and generating a reference self referencing electrical beat signal;first optical elements for propagating the heterodyne reflectometry beamincident to a target material at a predetermined angle of incidence;second optical elements for propagating the self referencing beamincident to the target material at the predetermined angle of incidence;a second heterodyne detector for receiving a reflected heterodynereflectometry beam from the target layer and generating a measurementelectrical heterodyne beat signal and for receiving a reflected selfreferencing beam from the target layer and generating a measurement selfreferencing electrical beat signal; a heterodyne phase shift detectorfor detecting a heterodyne phase shift between the reference heterodynereflectometry phase signal and the measurement heterodyne reflectometryphase signal; and a self referencing phase shift detector for detectinga reference phase shift between the reference self referencing phasesignal and the measurement self referencing amplitude modulated phasesignal; and a phase shift calculator for receiving the heterodyne phasemeasurement and the reference phase measurement and calculating a phaseshift of the measurement signal induced by the target material, and athickness calculator for receiving the phase shift of the measurementsignal induced and calculating a thickness parameter for the targetlayer.
 19. The heterodyne reflectometer recited in claim 18, furthercomprising: a split frequency, dual polarized beam source for generatingthe heterodyne reflectometry beam and the self referencing beam.
 20. Theheterodyne reflectometer recited in claim 18, further comprising: asplit frequency, dual polarized beam source for generating theheterodyne reflectometry beam; and a dual amplitude, single frequencybeam source for generating the self referencing beam.
 21. The heterodynereflectometer recited in claim 19, wherein the split frequency, dualpolarized beam further comprises: a first linear polarized beamcomponent at a first linear polarization and oscillating at a firstfrequency; and a second linear polarized beam component at a secondpolarization and oscillating at a second frequency, the first frequencybeing unique from the second frequency.
 22. The heterodyne reflectometerrecited in claim 20, wherein the split frequency, dual polarized beamfurther comprises: a first linear polarized beam component at a firstlinear polarization and oscillating at a first frequency; and a secondlinear polarized beam component at a second polarization and oscillatingat a second frequency, the first frequency being unique from the secondfrequency; and wherein the single frequency, dual amplitude beam furthercomprises: a third linear polarized beam component at one of the firstpolarization and the second polarization, having a first amplitude andoscillating at a third frequency; and a fourth linear polarized beamcomponent at one of the first polarization and the second polarization,having a second amplitude and oscillating at the third frequency. 23.The self referencing heterodyne reflectometer recited in claim 18,wherein one of the first optical elements and the second first opticalelements is comprised of material based on thermal stress inducedbirefringence properties.
 24. The self referencing heterodynereflectometer recited in claim 23, wherein the material is one of afused silica, BK7 silica and SF57 silica.
 25. The heterodynereflectometer recited in claim 18, wherein the predetermined angle ofincidence is related to a refractive index for the target layer.
 26. Theheterodyne reflectometer recited in claim 18, wherein the predeterminedangle of incidence is a predetermined default angle.
 27. The heterodynereflectometer recited in claim 18, wherein the predetermined angle ofincidence approximates Brewster's angle for the target layer.
 28. Theself referencing heterodyne reflectometer recited in claim 19, whereinthe heterodyne reflectometry beam source generates an s-polarized beamcomponent at the first frequency and a p-polarized beam component at thesecond frequency.
 29. The self referencing heterodyne reflectometerrecited in claim 18, wherein the optical chopper propagates one of theheterodyne reflectometry beam and the self referencing beam whilesimultaneously obstructing the other of the heterodyne reflectometrybeam and the self referencing beam, thereby switching a measurement beamat the measurement detector between a heterodyne reflectometry operatingmode and a self reference operating mode.
 30. The self referencingheterodyne reflectometer recited in claim 29, further comprises: achopper controller for iterating between the heterodyne reflectometryoperating mode and the self reference operating mode, said choppercontroller being operationally coupled to the optical chopper andbetween the first heterodyne detector and both the heterodyne phaseshift detector and the self referencing phase shift detector and betweenthe second heterodyne detector both the heterodyne phase shift detectorand the self referencing phase shift detector, wherein in the selfreferencing operating mode, the optical chopper obstructs a path of theheterodyne reflectometry beam and said chopper controller instructs theself referencing phase shift detector to detect the reference phaseshift between the reference self referencing phase signal and themeasurement self referencing amplitude modulated phase signal, andwherein in heterodyne reflectometry operating mode, the optical chopperobstructs a path of the self referencing amplitude modulated beam andsaid chopper controller instructs the heterodyne phase shift detector todetect the heterodyne phase shift between the reference heterodynereflectometry phase signal and the measurement heterodyne reflectometryphase signal.
 31. The self referencing heterodyne reflectometer recitedin claim 30, further comprises: a thickness calculator for receivingphase shift information from the phase shift calculator and calculatinga thickness parameter for the target material.
 32. The self referencingheterodyne reflectometer recited in claim 31, further comprises: athreshold noise detector for monitoring a reference phase measurementfrom successive self reference operating modes and comparing changesbetween successive reference phase measurements to a predeterminedthreshold noise level.
 33. The self referencing heterodyne reflectometerrecited in claim 32, further comprises: a phase shift averager foraveraging successive phase shifts from the phase shift calculator basedon a noise determination of one of the reference detector and themeasurement detector.